The ammonium salt of N-nitrosophenylhydroxyamine (cupferron) is a known polymerization inhibitor but has several drawbacks including its limited solubility which makes it difficult to add to some process streams.
Other derivatives of N-nitrosophenylhydroxyamine are also known (Gros, et al., U.S. Pat. No. 3,426,063 and Varwig U.S. Pat. No. 4,772,740). Solvents must be employed to prepare usable solutions. These solvents often cause problems during use. Cupferron has limited solubility in solvents such as water and lower alcohols. Cupferron has the following limits of solubility at 25.degree. C. in the indicated solvents: water (12%), methanol (5.5%) and isopropanol (0.38%), while it is essentially insoluble in hydrophobic solvent i.e., solvents which are immiscible with water. As a result of the limited solubilities of cupferron in the foregoing solvents, objectionably large amounts of these solvents are required in many applications where cupferron would otherwise be a desirable inhibitor.
Solutions of cupferron undergo degradation in the presence of air, as manifested by discoloration and formation of a black precipitate. Minimizing the extent and rate of degradation requires storing such solutions under an inert atmosphere such as nitrogen.
The above-mentioned deficiencies of cupferron solutions are not overcome by the salts of N-nitrosophenylhydroxylamine disclosed in the above-cited Gros patent. On the contrary, these salts of N-nitrosophenylhydroxylamine (NPHA) are unsatisfactory for use in inhibiting polymer formation in acrylic acids and acrylate esters. In the presence of such monomers, the amine salts decompose with formation of the organic aliphatic amines (e.g. ethylamine, etc.). The lower boiling amines (e.g. the C.sub.1 to C.sub.7 aliphatic amines) formed by decomposition of the corresponding amine salts create a substantial risk of co-distillation thereof with the acrylic acid or acrylate ester monomers being purified by distillation and resulting discoloration of polymers prepared from the monomers, such as, for example, poly(acrylic acid), poly(methyl methacrylate) and poly(ethyl acrylate). The higher boiling amines (e.g. the C.sub.8 -C.sub.20 aliphatic amines) formed upon decomposition of the corresponding amine salts are so immiscible with water that such salts are not entirely satisfactory for addition to water-containing acrylic acid systems.
Accordingly there is a need for a salt of NPHA which can be added to acrylic acid and acrylate ester systems and is effective for inhibiting undesired formation of polymer in such systems.